I wonder, but do not know, the motivation or the mental state of Eddie Ray Routh, the 25-year old former Marine now being held for the Saturday murders in Texas of former Navy SEAL sniper Chris Kyle and fellow military veteran Chad Littlefield.

But I want to advance some possible theories. In time, we may discover more.

The obvious one to the public, and the one getting all of the attention so far, is post-traumatic stress disorder.

PTSD is technically an anxiety disorder. The definition is changing at this time, but as it stands it is defined like this:

– There is a traumatic experience, accompanied by feelings of fear and helplessness.

– There is a cluster of symptoms, including re-experiencing intrusive memories, numbness, and exaggerated reactions.

– There is an impairment in functioning.

Another possibility is psychosis.

– Psychosis, by definition, is a lack of contact with reality, usually accompanied by delusions or hallucinations. The most common mental disorder with psychosis is schizophrenia, which often presents in the early 20s.

Substance abuse is a third possible cause.

Alcohol is, of course, commonly abused. However, other substances, such as PCP, methamphetamines, and the newer synthetic marijuana (K2, spice, etc.) often cause psychosis.

How to sort these out?

Normally there will be an evaluation by a psychiatrist or psychologist, especially trained in this area (a “forensically trained psychiatrist or psychologist”). He or she will do an evaluation to find out whether PTSD, psychosis, substance abuse, or a combination, may have played a role in the accused mindset at the time of the crime.

The evaluation will consist of a clinical interview, gaining information from other sources such as family and friends, and may include psychological testing. The latter may include the ink-blot Rorschach test, the MMPI (the Minnesota Multiphasic Personality Inventory) test, and other tools.

I am a forensic psychiatrist. I try to figure out whether someone is mentally ill, and – if so — whether they should be judged not criminally responsible or not guilty by reason of insanity due to such impairments.